1. Technical Field
The present invention relates, in general, to desensitization, and, in particular, to a method of inhibiting agonist-specific desensitization, to compounds suitable for use in such a method and to pharmaceutical compositions comprising same.
2. Background Information
Desensitization is a general phenomenon which is characterized by a reduced responsiveness following prolonged exposure to a hormone or drug. Occupancy of a wide variety of hormone and neurotransmitter receptors by agonists often leads to desensitization, that is, to a loss of receptor responsiveness to subsequent stimulation by agonist. This particular phenomenon is generally termed homologous desensitization. This is in contrast to the heterologous form of desensitization, which is defined as a loss of receptor responsiveness caused by agonist occupancy of other receptors.
Homologous desensitization has been most thoroughly studied for the xcex2-adrenergic receptor (xcex2AR)-adenylyl cyclase system (Benovic et al (1988) Ann. Rev. Cell Biol. 4:405-428). Homologous desensitization of xcex2ARs is accompanied by receptor phosphorylation (Sibley et al (1985) J. Biol. Chem. 260:3883-3886; Strasser et al (1986) Biochemistry 25:1371-1377). A cAMP-independent kinase, termed xcex2AR kinase, has been described that specifically phosphorylates the agonist-occupied forms of the xcex22-adrenergic receptor (xcex22AR) and xcex12-adrenergic receptor (Benovic et al (1986) Proc. Natl. Acad. Sci. USA 83:2797-2801; Benovic et al (1987) J. Biol. Chem. 262:17251-17253) as well as light-activated rhodopsin (Benovic et al (1986) Nature (London) 322:867-872). Phosphorylation of the xcex22AR by xcex2AR kinase may trigger the process of functional uncoupling from the stimulatory guanine nucleotide binding protein, Gs (Benovic et al (1987) Proc. Natl. Acad. Sci. USA 84:8879-8882).
While there have been several publications involving attempts at blocking as desensitization (Mallorga et al (1980) Proc. Natl. Acad. Sci. USA 77:1341-1345; Shima et al (1983) J. Biol. Chem. 258:2083-2086; Heyworth et al (1984) Biochem. J. 222:189-194; Fano et al (1986) J. Auton. Pharmacol. 6:47-51; DeBernardi et al (1987) Proc. Natl. Acad. Sci. USA 84:2246-2250; and Toews (1987) Mol. Pharmacol. 32:737-742), none of these studies directly demonstrates the inhibition of agonist-specific desensitization. Moreover, none of the compounds utilized in these studies directly inhibit the xcex2AR kinase. The identification of compounds which specifically inhibit this kinase could provide a method of inhibiting desensitization. The inhibition of desensitization should result in enhanced and prolonged receptor action in response to endogenous compounds and drugs. In the case of drugs, this means increased drug efficacy leading to the use of lower drug doses, and reduced side effects. In addition, it should allow prolonged treatment with drugs.
It is a general object of the invention to provide a method of blocking agonist-specific desensitization.
It is a specific object of the invention to provide compounds capable of inhibiting agonist-specific desensitization.
It is a further object of the invention to provide a pharmaceutical composition comprising as an active ingredient a compound capable of blocking agonist-specific desensitization.
It is another object of the invention to provide a method of augmenting the efficacy and duration of treatment of drugs, the continued administration of which results in desensitization.
It is a further object of the invention to provide a method of restoring the effectiveness of receptor-mediated responses to endogenous compounds such as hormones and neurotransmitters.
Further objects and advantages of the present invention will become clear to one skilled in the art from a reading of the description that follows.
In one embodiment, the present invention relates to a method of inhibiting desensitization of a cell to the effects of a compound. The method comprises contacting the cell with an agent capable of inhibiting phosphorylation, by a protein kinase, of a receptor for the compound present on the surface of the cell.
In another embodiment, the present invention relates to a method of screening a compound for its ability to inhibit desensitization. The method comprises:
i) contacting a receptor specific kinase-containing sample with the compound under conditions such that interaction between receptor specific kinase present in the sample and the compound can occur, and
ii) determining the ability of the receptor specific kinase to phosphorylate the receptor for which it is specific.
In a further embodiment, the present invention relates to an inhibitor of xcex2 adrenergic receptor kinase. The inhibitor consists essentially of a peptide corresponding to an intracellular domain of the xcex22 adrenergic receptor.
In yet another embodiment, the present invention relates to a pharmaceutical composition. The composition comprises as an active ingredient the above-described peptide (or other agent) in an amount sufficient to exhibit an inhibitory effect on xcex2 adrenergic receptor kinase, together with a pharmaceutically acceptable carrier.